ABSTRACT: Species in the Mytilus edulis complex (M. edulis, M. galloprovincialis and M. trossulus) hybridize in nature but, depending on the species pair involved, hybridization can range from minimal to very common with extensive introgression. Since M. galloprovincialis has been introduced within the geographic range of M. trossulus in several locations, this system provides the opportunity to determine whether the propensity for hybridization and the resulting level of introgression depends upon environmental circumstances in which the 2 species become sympatric. Contact between these 2 species on the Northeast Pacific coast and within the Puget Sound results in low levels of hybridization with little to no introgression. The present study examines the level of hybridization and introgression between M. galloprovincialis and M. trossulus around Hokkaido, Japan, where these species are also sympatric. We show that 2 independent hybrid zones exist in Hokkaido, each having distinct genetic architectures. The hybrid zone formed along the northern coast of Hokkaido is similar in structure to those reported in the northeast Pacific. The second hybrid zone formed along the southern coast of Hokkaido differs markedly from these other hybrid zones by exhibiting an exceptionally high propensity for hybridization, but, like other hybrid zones between these 2 species, it generates minimal levels of introgression. The mechanism(s) producing high levels of hybridization between M. galloprovincialis and M. trossulus in southern Hokkaido may include some form of apparent outcrossing or hybrid vigor for survival but not for fertility.